1. Field of the Invention
This invention relates to a sheet feeding device for use in copying machines, various printers, facsimile transmitters and so on, and more particularly to an automatic sheet feeding device provided with a sheet separation means capable of completely preventing simultaneous feeding of overlapping sheets of paper or documents to reliably send out the sheets stacked on a sheet stacker one by one to a prescribed position in the copying machine or the like.
2. Description of the Prior Art
In image- or word-processing devices including printers of various types, copying machines, facsimile transmitters or the like in which copying or printing sheets of paper or sheet documents contained in a sheet stacker or cassette in a piled state are fed one by one to a prescribed position, a sheet sending-out stage is essentially provided with a sheet separation means for separating the overlapping sheets so as to allow only one sheet to pass therethrough. Thus, with the sheet separation means, simultaneous feeding of two or more overlapping sheets of paper, what is called "a double-feed phenomenon" can be prevented.
Conventionally there has been a sheet separation means of this type which comprises a sheet feeding roller and at least one friction member being in contact with the sheet feeding roller. The incorporated sheet feeding roller and friction member are generally disposed in close proximity to a sheet stacker in which a plurality of sheets of paper are contained in the piled state. When two sheets of paper are discharged from the sheet stacker and intrude into between the sheet feeding roller and friction member, one of the sheets which comes into contact with the friction member is prevented from advancing by the friction brought about by the friction member and the other sheet coming into contact with the sheet feeding roller is fed forward by the rotation of the roller. As a result, only one sheet is sent out toward the prescribed position.
Some of sheet separation means of the type that a friction pad or plate formed of rubber or other elastic material is used as the friction member noted above and comes into contact with a sheet feeding roller have been so far proposed in U.S. Pat. Nos. 4,368,880, 4,674,737 and 4,696,462, for instance.
Moreover, there have been sheet separation means of different types as disclosed in U.S. Pat. Nos. 4,085,929, 4,114,870 and 4,544,147. These conventional sheet separation means by and large employ a friction member which moves or rotates in the opposite direction to the rotation of a sheet feeding roller being in contact with the friction member in order to improve the sheet separating effect. In some of the prior art, this movable friction member is used jointly with the friction pad or plate touched upon above.
However, the conventional sheet separation means cannot necessarily bring about the effect of reliably separating the overlapping sheets sent out from a stack of sheets. This is because the sheet feeding roller and friction member should be essentially retained in a remarkably delicate contacting state so as to feed only one sheet skillfully and simultaneously avoid skidding of the sheet feeding roller (what is called "a non-feed phenomenon") which is caused by increasing the contact pressure of the friction member relative to the sheet to be fed, notwithstanding the intention of completely preventing the so-called double-feed phenomenon. Consequently, if the conditions such as coefficient of friction of the surface of the sheet to be fed and stiffness of the sheet do not conform to the design conditions of the sheet separation means including the friction member, or in a case that the overlapping sheets are in an unexpected state, the double-feed phenomenon would be inevitably brought about. In other words, since the conventional sheet separation means are designed in conformity with the properties (stiffness, coefficient of friction of the surface, thickness of the sheet, etc) of a specified sheet to be used, the aforementioned double-feed and non-feed phenomena are inevitably caused when using a sheet which is no match for the specified sheet. Such a disadvantage is due to the contacting condition of the sheet feeding roller and friction member, which condition is not changeable with the properties of the sheet to be fed.
Though the conventional sheet separation means as earlier given as the latter prior art employing the movable friction member which moves or rotates in the opposite direction to the rotation of the sheet feeding roller can relatively heighten the effect of separating the overlapping sheets, it is unsuitable for a thin sheet of paper and, over and above, becomes complicated in mechanism. If the conventional sheet separation means is so designed as to deal with the thin sheet of paper, it entails a problem such that a thick sheet of paper cannot be fed successfully. Some of the aforementioned prior art employ multi-stages of sheet separation means in order to remedy the drawbacks of the conventional sheet separation means as noted above, or otherwise, use a gate disposed at an access portion to the sheet separation means in order for allowing only a sheet having thickness smaller than a predetermined specific thickness to pass therethrough. However, there has not been hitherto proposed a sheet feeding device provided with a sheet separation means capable of dealing with any sheet of paper whatever and reliably separating the overlapping sheets so as to feed the stacked sheets one by one to a prescribed position.
In the sheet separation means in which the friction pad is in pressure contact with the sheet feeding roller as described in U.S. Pat. No. 4,368,880, the friction pad is swingingly supported by a retaining arm having its one end pivoted at a supporting point at a distance from the point of contact between the friction pad and the feeding roller, so that the friction pad moves away from the feeding roller about the supporting point when a sheet of paper enters between the pad and roller. Hence, since the friction pad leaves apart from the roller when the sheet of paper is fed, the space between the pad and roller is merely increased. As a result, this prior art has suffered a disadvantage that two or more sheets of paper easily intrude in therebetween and the so-called "double-feed" is susceptible to occur.